1. Field of the Invention
The present invention relates to a method of crystallizing a silicon film and a method of manufacturing a liquid crystal display, and more particularly, to a method of crystallizing a silicon film using a laser and a method of manufacturing a thin film transistor liquid crystal display using the same.
2. Description of the Related Art
A thin film transistor (TFT) is mainly used as a switching device of a liquid crystal display (LCD). A polycrystalline silicon forming a semiconductor layer used as a channel of the TFT is formed by crystallizing an amorphous silicon film.
The most widely used method of crystallizing the amorphous silicon film is an annealing method using a pulsed UV beam, i.e., an excimer laser. A laser annealing has been developed for annealing silicon into which impurity ions are implanted in a large scale integration (LSI) process. This method has been applied to the development of a display device having a large area. Recently, it began to be applied to the manufacturing of low temperature polycrystalline silicon TFT-LCD products having medium and small sizes. The method of manufacturing a good quality of polycrystalline silicon by annealing the amorphous silicon thin film using a laser has an advantage in that a substrate is not damaged since thermal annealing is performed within a short time though a melting temperature is high.
The change of phase from the amorphous silicon to the polycrystalline silicon is performed through two physical mechanisms such as a nucleation in which a crystal is formed and a growth in which the generated crystal is grown. Therefore, if it is possible to suppress the nucleation and to allow the growth to occur after the amorphous silicon is completely melted, the method is suitable for improving the crystal structure of the polycrystalline silicon.
FIG. 1 is a sectional view describing a method of crystallizing the silicon film using conventional laser annealing. Reference numerals 10 and 15 respectively denote a transparent substrate and an amorphous silicon film formed on the transparent substrate 10.
According to the conventional method of crystallizing the silicon film, the crystallization is performed by irradiating a laser on amorphous silicon film 15, thus temporarily melting and cooling the silicon film. At this time, the melting temperature of the amorphous silicon film and the state of the crystallization vary according to the energy intensity of the irradiated laser. For example, with a higher energy intensity of the irradiated laser, the deeper amorphous silicon film is melted from the surface. As the energy intensity increases, the amount of melted silicon film increases. The amorphous silicon film is completely melted beyond a predetermined threshold energy intensity. The grain size of the crystallized silicon is proportionaal to the energy intensity of the irradiated laser. Namely, as more amorphous silicon film is melted, the grain size increases. This is because small grains are generated during a process in which only the surface of the amorphous silicon film is melted by the energy intensity less than the threshold energy and is cooled. Since a small amount of amorphous silicon film is left and the rest of the silicon film is nearly completely melted by the energy intensity of the laser approximate to the threshold energy intensity, the silicon film which is not melted operates as a seed, thus crystallizing a larger grain. When the amorphous silicon film is completely melted by heightening the energy intensity of the laser to higher than the threshold energy intensity, the grain size is reduced since there is no silicon film left which will operate as a seed and the crystallization is performed on the basis of irregular nucleation and growth.
In general, in order to manufacture a TFT device having excellent performance, the size of the crystal grain of the polycrystalline silicon must be large and the defect intensity and the surface roughness of the crystal must be small. In particular, the crystal grains and the crystal defects operate as a scattering factor with respect to the movement of a charge carrier, thus deteriorating field effect mobility. This is why a method of leaving only the amorphous silicon film which operates as the seed by approximating the energy intensity of the laser to the threshold energy intensity was used in a conventional technology.
However, since the range of energy intensity in which a large grain can be obtained is very narrow in the conventional method, the width of an allowable margin is very narrow during the process. Also, since grains are irregularly located in the polycrystalline silicon crystallized by the conventional method, it is difficult to secure a uniform element characteristic when the polycrystalline silicon is used as a semiconductor layer in which the channel area of the TFT is formed.
As shown in FIG. 2, an LCD is comprised of a pixel portion and a driving circuit portion.
Referring to FIG. 2, an LCD panel for displaying a picture is comprised of a pixel portion in which a thin film transistor and a pixel electrode are arranged and a driving circuit located around the pixel portion for driving the thin film transistor and realizing a picture in the pixel portion.
The TFT is formed in the pixel portion and the driving circuit portion. However, the crystallization is not considered as an important factor in the TFT of the pixel portion since a large area can be easily driven by satisfying a mobility of about 20–50 cm2/Vs. However, a high uniformity is required to reduce leakage current.
An N channel TFT and a P channel TFT must be formed in the driving circuit portion. Since the characteristics of the TFTs affect the driving of the LCD panel, the polycrystalline silicon film used for the driving circuit preferably has a single crystal.
In the conventional technology, one silicon film was used for the TFT of the pixel portion and the TFT of the driving circuit portion. It was difficult to satisfy the need of the TFT of the driving circuit portion in which the crystallization which affects the characteristic of the TFT is important and the need of the TFT of the pixel portion in which the uniformity is more important than the crystallization at the same time. Therefore, it was difficult to manufacture an improved LCD.